1) Field of the Invention
This invention relates generally to fabrication of semiconductor devices, and to the manufacture of integrated circuits, and more particularly to the formation of device interconnections on semiconductor integrated circuits by etching of a metal layer with gaseous reagents assisted by electrical plasma excitation of the reactive medium and to methods of selectively removing sidewall polymers from metal lines.
2) Description of the Prior Art
The density of devices fabricated on semiconductor substrates has increased steadily over the years with ultra large scale integration. Accompanying this trend have been decreased feature sizes and increased demands on process technology. Patterns of fine conductive lines are required to interconnect electronic devices thereon, in order to carry electrical signals and to distribute electrical current and power to the devices. It is very desirable to use metals of high electrical conductivity for this purpose to minimize power loss and undesirable heating. The preferred materials for this purpose are aluminum and its alloys, particularly with copper. The economic benefit from reducing overall device dimensions derives from the lower unit cost per device if more devices can be fabricated per unit area of semiconductor substrate. The desire for finer lines follows directly, and has resulted in the development of methods for accurate and precise fabrication of metallic linewidths and spacings of the order of several microns. This is accomplished by methods of etching of the metallic layers by means of gas-phase removal of the metallic material selectively exposed by an appropriate photoresist pattern. This process is often enhanced and improved by carrying it out in the presence of an electrical plasma sustained in the reactive gaseous etching mixture by input of radio frequency power to the reaction chamber. The desired high rates and selectivity of etching of aluminum and its alloys are readily attained by use of gaseous compounds containing chlorine such as chloroform, for example.
In addition, the resist is difficult to remove as a result of larger amounts of etch byproducts such as sidewall polymer on vertical walls of a device undergoing fabrication. These byproducts, generally referred to as polymers are generally comprised of a metal and SiO2 molecule. For instance, the molecule can comprise carbon from the photoresist, metal from the metal layer and SiO2. Further, sidewall polymers may comprise aluminum silicate and very small amounts of fluorocarbons. Fluorocarbons are non-combustible and therefor are not removed during an O2 in-situ ash sequence of a metal etch. Thus, ashing has proven to be ineffective because of the high carbon content in the byproduct molecule from the photoresist. The difficulty with which resist can be removed has proven to be a sever impediment to the generation of sub-half micron features. Previously solvent/ultrasonic agitation had been used to remove SWP. However, these techniques prove to be unusable because of the tendency of metal, such as aluminum, to lift off of the minimum features. Further, these techniques tend to leave behind significant amounts of residue on device sidewalls and on device surfaces.
For the reasons cited, it has been found beneficial to carry out a post-etching procedure subsequent to metal pattern etching. Removal of the device substrates from the reaction chamber followed by exposure to solvent or aqueous cleaning media has been one solution. Another procedure has been a subsequent exposure of the devices to a RF plasma sustained in an oxygen (O2) gas environment, which has been found to be beneficial in removing residues containing trapped Cl-containing species, and in minimizing the amount of intractable polymer surface skin on the photoresist which interferes with the stripping of the latter material. It has been reported that the use of fluorine-containing gases provides a suitable post-etching treatment for reducing Cl-containing residues after gas-phase etching of aluminum. It is thought that this process substitutes F atoms for the Cl atoms in the residues, or coats the residues with an impervious fresh polymer and limits access to the trapped Cl atoms by water.
The importance of overcoming the various deficiencies noted above is evidenced by the extensive technological development directed to the subject, as documented by the relevant patent and technical literature. The closest and apparently more relevant technical developments in the patent literature can be gleaned by considering U.S. Pat. No. 5,851,302(Solis) shows a method for dry etching sidewall polymer.
U.S. Pat. No. 5,348,619(Bohannon et al.) teaches a metal selective polymer removal process.
U.S. Pat. No. 5,755,891(Lo et al.) shows a post metal etch process.
U.S. Pat. No. 5,814,155(Solis et al.) shows plasma ashing polymer/photoresist removal processes.
It is an object of the present invention to provide a method for etching a metal line and removing the polymer that is formed on the sidewalls of the metal lines.
It is an object of the present invention to provide a method for etching a Aluminum alloy metal line and removing the polymer that is formed on the sidewalls of the metal lines using a chlorine containing (e.g., Cl2) and fluorine containing (e.g., CF4) post etch polymer removal step.
It is an object of the present invention to provide a method for etching a aluminum alloy metal line having a Ti/TiN anti-reflective coating layer and a Ti underlayer and removing the polymer that is formed on the sidewalls of the metal lines using a chlorine containing (e.g., Cl2) and fluorine containing (e.g., CF4) post etch clean step.
To accomplish the above objectives, the present invention provides a method of patterning a metal line and removing the polymer layer that forms on the metal lines sidewalls in a post etch-clean step. The method can be summarized as follows. A semiconductor structure and an overlying dielectric layer are provided over the dielectric layer, a first barrier layer, a metal layer; a second barrier layer are provided. A resist pattern over the second barrier layer is provided. We perform a four step etch process, in sequence in the same etch chamber. In a first etch step (A); we etch through the second barrier layer using a B and Cl containing gas and a Cl containing gas in a reactive ion etch to form a first polymer layer over the sidewall of the second barrier layer. In a second etch step (B); the metal layer is etched exposing the first barrier layer to form a second polymer over the first polymer and the sidewall of the metal layer; the second etch step performed using a B and Cl containing gas and a Cl containing gas. In a third etch,step (C); the first barrier layer is etched to form a third polymer layer over the first and second polymer layers; the third etch step performed using a B and Cl containing gas and a Cl containing gas. In an important fourth etch step (D); we remove the first, second and third polymers; the fourth etch step performed using only chlorine containing gas (Cl2) gas and a fluorocarbon containing gas.
The invention""s etch process removes the polymer on the sidewalls of metal lines. The key fourth step removes the sidewall polymer layer(s).
The present invention achieves these benefits in the context of known process technology. However, a further understanding of the nature and advantages of the present invention may be realized by reference to the latter portions of the specification and attached drawings.